Evolution of interface states and stick-slip mechanism of lightweight friction pairs with varying braking pressures

Stick-slip vibration in lightweight friction pairs used in urban rail trains significantly degrades braking performance. Brake pressure, a key factor influencing stick-slip vibration, has an unclear mechanism of action. This study reproduced stick-slip phenomena through braking pressure gradient tests (0.25–0.75 MPa) and established relationships among braking pressure, friction interface states, and stick-slip vibrations. The evolution of stick-slip behavior governed by braking pressure was systematically investigated. Results show that the stick-slip amplitude increases significantly as braking pressure rises from 0.25 MPa to 0.50 MPa. This increase is attributed to the formation of stable tribofilms and oxide layers on the friction interface, reduced surface roughness, and increased variability in contact plateaus, which together enhance both tangential and normal stiffness. Conversely, within the 0.50–0.75 MPa range, further increases in pressure damage the tribolayer's structural integrity, producing flaky wear debris that accumulates on the contact surface. These changes reduce contact stiffness between the brake disc and pad, diminishing stick-slip amplitude. Overall, stick-slip amplitude shows a non-monotonic variation with braking pressure, with an enhancement phase driven by interfacial strengthening and an attenuation phase caused by tribolayer degradation. This study provides a theoretical basis for the tribological behavior and stick-slip vibrations of lightweight friction pairs under varying braking pressures and offers guidance for optimizing pressure parameters to mitigate stick-slip vibrations in urban rail transit systems.